According to the microsoft database, it is impossible in practice to change the letter of the system partition (you can easily find it, I think).
This is a problem that can occur in case of migration of the installation to another hard drive. If you duplicate the partition to a new hard drive and then boot into windows with both drives connected, the system detects the new drive and partitions and registers them. As it isn't allowable to have a letter corresponding to two partitions at the sane time, the new one is named by the first free letter. These letters are kept by the system registry in many keys, so it is not possible in practice to everything fix.
The best practice is to duplicate the hard drive (or copy the partitions to a new drive) with a Linux tool, remove the old drive, install the new one in the place of the old drive and then reboot into the original system.
Simply resizing a partition doesn't affect partition letters, so I guess you did copies of the partitions.
That program installer obviously doesn't provide any option to use other than C: partition.
I'm afraid you have to reinstall the o.s. to fix the problem, unless you find anything in some mswindows tech forum.
Another idea, if you have some free space on the drive (or on any other hard drive on the computer): create a new small partition that you name C: (this would be possible if you don't have already any C: partition in use), to see if this can solve the problem.
*** It is highly recommended to backup any important files before doing resize/move operations. ***
